Optical Frequency Band Research Articles

SAL vibration estimation and compensation based on triangular interferometric signals.

Synthetic aperture Ladar (SAL) is an extension of synthetic aperture technology in the optical frequency band. Owing to the short wavelength of lasers, the system has high-resolution, high-data-rate, and refined imaging capabilities, which has potential in high-resolution observation fields such as ground observation and space target observation. However, the short wavelength of lasers also makes SAL severely sensitive to vibrations even on the micron order which cause azimuth defocusing and range cell migration. To address this problem, we establish a de-chirp signal model under vibration environment, and propose a vibration error estimation and compensation method using triangular interferometric signals. According to the symmetrical characteristics of triangular frequency modulated continuous wave (T-FMCW) and the time-frequency information introduced by the azimuthal vibration phase, we use a two-stage interferometry method to estimate instantaneous frequency introduced by the vibration errors that cause range cell migration. For the scenarios without obvious range cell migration, we use a one-stage interferometry method to estimate the instantaneous frequency. Subsequently, we establish a vibration compensation filter using the estimated instantaneous frequency to compensate for the vibration errors. We use two experiments to verify the effectiveness and superiority of the proposed method. The results show that the proposed method effectively eliminates range cell migration and azimuthal phase errors introduced by vibration errors, producing SAL imaging results with higher resolution than the conventional spectral correlation method.

Tune-out wavelengths of Rydberg atoms

The atomic polarizability represents the response characteristics of atoms to externally applied electro-magnetic fields. The wavelength (or frequency) at which the dynamic polarizability of an atom is equal to zero is referred to as the tune-out wavelength (or frequency). Spectroscopy technology based on the tune-out effect has potential applications in quantum precision measurement, quantum computation and quantum communication. Related research topics include the measurement of fundamental physical constants and strong interactions. The tune-out wavelengths of atoms in low-lying states primarily fall within the optical band, where the theoretical calculations and experimental measurements have significant progress. However, for Rydberg atoms in highly excited states, theoretical calculations are challenging due to their high density of atomic states. The difficulty of experimental measurement arises from small splitting of adjacent atomic energy levels. In this paper, we demonstrate the tune-out wavelengths measurement for Rydberg atoms in a cesium vapor cell at room temperature. We utilize a two-photon cascade excitation to prepare Rydberg states and employ amplitude-modulation electromagnetically-induced transparency (AM-EIT) spectroscopy to measure the tune-out wavelength. By continuously scanning the microwave frequencies, we obtain AM-EIT signals of Rydberg atoms. At near-resonant microwave transition wavelengths, strong AM-EIT signals are observed due to microwave-atom coupling. Conversely, at tune-out wavelengths, the dynamically polarization-induced destructive interference in neighboring energy states occurs which leads to the weak AM-EIT signals. The AM-EIT provides a spectral resolution of about 10 MHz. We have developed a simplified three-level model to calculate the tune-out wavelength. The results of our theoretical calculations are consistent with the experimental findings within a range of ±90 MHz.

A Microwave Field-Induced Nonlinear Metamaterial with Wafer Integration Level.

Field-induced nonlinear materials, with extended abilities of manipulating electromagnetic waves, have been widely employed in electromagnetic protection, absorption, and detection. Until now, it was found that the field-induced nonlinearity mainly shows in the optical and terahertz frequency bands. Applying the microwave band into such technical activities is hampered due to a lack of investigations on the nonlinearity caused by microwave electric fields, especially in the ultrawideband and microwave high-frequency bands. In this paper, a nonlinear metamaterial (NLMM) concept based on the integration of metamaterial structures and a semiconductor on the same wafer is proposed, which shows nonlinear behavior to the electromagnetics' field energy in the microwave band. The designed NLMM is transparent to low-density electromagnetic radiation fields, while it adaptively becomes opaque to high-density electromagnetic radiation fields. Two types of NLMM are designed to verify the nonlinear characteristics of ultrawide and narrow bands in the microwave band, respectively. The concept of NLMM can be used for the application of the microwave frequency band in electromagnetic protection and detection.

Non-reciprocal frequency conversion in a two-dimensional waveguide incorporating a local nonlinear gate

We study non-reciprocal frequency conversion in a linear two-dimensional (2D) acoustic waveguide consisting of coupled oscillators incorporating a local asymmetric gate of nonlinearly coupled oscillators. The 2D waveguide supports an acoustic and an optical frequency band. We consider a monochromatic harmonic excitation at the acoustic band that initiates a propagating wave in the linear waveguide. As the wave scatters across the nonlinear gate, its frequency is converted to a higher (triple) harmonic in the optical band. This frequency conversion is due to a local 1:3 internal resonance at the nonlinear gate. The efficacy and robustness of the frequency conversion are validated by direct numerical simulations. A large portion of the energy of the wave is transmitted across the nonlinear gate with converted frequency and this frequency conversion is realized robustly over a range of excitation frequencies and magnitudes. In addition, owing to the nonlinearity, the frequency conversion is tunable to frequency and energy. The transmissibility (i.e., transmitted over reflected energy at the gate) is sensitive to the excitation parameters indicative of bifurcations of the nonlinear acoustics. Lastly, frequency conversion induces strong non-reciprocity in the acoustics since the energy transfer with frequency conversion is only realizable in a preferred direction of wave propagation and prohibited in the reverse direction. The methodology and results of the present nonlinear frequency converter are applicable to various fields including targeted energy transfer in phononic systems and acoustic computing.

Ultrawideband metamaterial absorber for oblique incidence using characteristic mode analysis

An ultrawideband, polarization-insensitive, metamaterial absorber for oblique angle of incidence is presented using characteristic mode analysis. The absorber consists of conductive meander square loops and symmetric bent metallic strips, which are embedded with lumped resistors. With the aid of modal currents and modal weighting coefficients, the positions of the lumped resistors are determined. After that, the equivalent circuit (EC) model and admittance formula are proposed and analyzed to further understand the working principle and ultrawide bandwidth. The proposed absorber measures an absorption bandwidth of 4.3–26.5 GHz (144.1% in fractional bandwidth) for 90% absorptivity under normal incidence. At the oblique angle of incidence of 45°, the bandwidth of 90% absorptivity is still 5.1–21.3 GHz (122.72%) for transverse electric (TE) polarization, and 6.8–29.5 GHz (125.07%) for transverse magnetic (TM) polarization. The good agreement among simulation, measurement, and EC calculation demonstrates the validity of the proposed method and indicates that the method can be applied to other microwave and optical frequency bands. The proposed metamaterial absorber can be widely applied in electromagnetic compatibility, electromagnetic interference, radar stealth, and biomedical detection.

Dual-polarization multi-angle retroreflective metasurface with bilateral transmission windows.

Metasurfaces have provided unprecedented degrees of freedom in manipulating electromagnetic (EM) waves and also granted high possibility of integrating multiple functions into one single meta-device. In this paper, we propose to incorporate the retroreflection function with transmission function by means of metasurface design and then demonstrate a dual-polarization multi-angle retroreflective metasurface (DMRM) with bilateral transmission bands. To achieve high-efficiency retroreflections, the compact bend structures (CBSs), which exhibit high reflections around 10.0 GHz in X band, are added onto the substrate of the DMRM. Two selected metasurface elements are periodically arranged so as to form 0-π-0 phase profile. By delicately adjusting the periodicity, high-efficiency retroreflections can be produced for both TE and TM-polarized waves under both vertical incidence and oblique incident angles ±50.0°, with an average efficiency of 90.2% at the designed frequency. Meanwhile, the two metasurface elements exhibit high transmission properties and minor phase disparities in S, C and Ku bands, resulting in bilateral transmission windows. Prototypes were designed and fabricated. Both simulated and measured results verified our design. This work provides an effective means of integrating retroreflection functions with other functions and may find applications in target tracking, radomes and other sensor integrated devices in higher frequency or even optical frequency bands.

Nonreciprocal Waveguide-QED for Spinning Cavities with Multiple Coupling Points

We investigate chiral emission and the single-photon scattering of spinning cavities coupled to a meandering waveguide at multiple coupling points. It is shown that nonreciprocal photon transmissions occur in the cavities-waveguide system, which stems from interference effects among different coupling points, and frequency shifts induced by the Sagnac effect. The nonlocal interference is akin to the mechanism in giant atoms. In the single-cavity setup, by optimizing the spinning velocity and number of coupling points, the chiral factor can approach 1, and the chiral direction can be freely switched. Moreover, destructive interference gives rise to the complete photon transmission in one direction over the whole optical frequency band, with no analogy in other quantum setups. In the multiple-cavity system, we also investigate the photon transport properties. The results indicate a directional information flow between different nodes. Our proposal provides a novel way to achieve quantum nonreciprocal devices, which can be applied in large-scale quantum chiral networks with optical waveguides.

Mixed-Carrier Communication for Technology Division Multiplexing

Recently, research on sixth-generation (6G) networks has gained significant interest. 6G is expected to enable a wide-range of applications that fifth-generation (5G) networks will not be able to serve reliably, such as tactile Internet. Additionally, 6G is expected to offer Terabits per second (Tbps) data rates, 10 times lower latency, and near 100% coverage, compared to 5G. Thus, 6G is expected to expand across all available spectrums including terahertz (THz) and optical frequency bands. In this manuscript, mixed-carrier communication (MCC) is investigated as a novel physical layer (PHY) design for 6G networks. The proposed MCC version in this study is based on visible light communication (VLC). MCC enables a unified transmission PHY design to connect devices with different complexities, simultaneously. The design trade-offs and the required signal-to-noise ratio (SNR) per individual modulation schemes embedded within MCC are investigated. The complexity analysis shows that a conventional optical OFDM receiver can capture the high-speed bit-stream embedded within MCC. For a forward error correction (FEC) bit-error-rate (BER) threshold of 3.8×10−3, MCC is optimized to maximize the spectral efficiency by embedding 2-beacon phase-shift keying (2-BnPSK) within an MCC envelope on top of 12 bits per beacon position modulation (BPM) symbol.

Molecular Dynamic Investigation of the Anisotropic Response of Aluminum Surface by Ions Beam Sputtering

Aluminum optics are widely used in modern optical systems because of their high specific stiffness and high reflectance. With the applied optical frequency band moving to visible, traditional processing technology cannot meet the processing precision. Ion beam sputtering (IBS) provides a highly deterministic technology for high-precision aluminum optics fabrication. However, the surface quality is deteriorated after IBS. The interaction between the bombard atoms and the surface morphology evolution mechanism are not clear, and systematic research is needed. Thus, in this paper, the IBS process for single crystal aluminum with different crystallographic orientations are studied by the molecular dynamics method. The ion beam sputter process is firstly demonstrated. Then, the variation of sputter yield of the three crystal faces is analyzed. The sputter yield difference of different crystal surfaces causes the appearance of the relief structure. Then, the gravel structure generates on the single crystal surfaces and dominates the morphology evolution. The state of the atom diffusion of the specific crystal surfaces will determine the form of the gravel structure. Furthermore, the form and distribution of subsurface damage and stress distribution of three different crystal surfaces are analyzed. Although there are great differences in defect distribution, no stress concentration was found in three workpieces, which verifies that the ion beam sputter is a stress-free machining method. The process of IBS and the mechanism of morphology evolution of aluminum are revealed. The regularity and mechanism will provide a guidance for the application of IBS in aluminum optics manufacture fields.

Design and Analysis of Plasmonic Antenna for Nanoscale Wireless Applications

The paper demonstrates design and analysis of plasmonic antenna for nanoscale wireless links. A surface plasmon polarities (SPP’s) based Hybrid Metal Insulator Metal (HMIM) based circular nano patch antenna for optical frequencies are designed and analysed using commercially available CST Microwave studio suite. The proposed circular patch antenna operated in the optical frequency band of 225 THz to 245 THz with a bandwidth of 20 THz. The overall size of the optical antenna is 1500 x 1000 nm2. The proposed antenna is very much useful in chip scale applications, interconnects and optical sensing applications.

ДОСЛІДЖЕННЯ НАНОКРИСТАЛІТІВ, ОТРИМАНИХ НА ОСНОВІ НАНОПОРИСТИХ МЕМБРАН AL2O3 ІЗ НАСИЧЕНИХ ВОДНИХ РОЗЧИНІВ KH2PO4

Розглядається питання отримання та використання нанокомпозитних матеріалів для радіомереж оптичної та терагерцової смуг частот. Приготовлені нанопористі структури анодованого оксиду алюмінію Аl2О3 (АОА) діаметром пор 40 та 75 нм. Отримано та проведено дослідження кристалічного матеріалу KH2PO4 (KDP) у вигляді нанопроводів та нанотрубок при різних варіантах росту на основі мембрани Al2O3 і насиченого водного розчину KDP.

Titanium Vanadium Oxide Thin Films Prepared by Thermal Oxidation of Titanium-Vanadium Alloy as Photocatalyst for Methylene Blue Degradation under Visible Light

In this work, titanium-vanadium oxide composite thin films were prepared by thermal oxidation of vanadium and titanium metallic alloys, at 500 °C under air atmosphere. The metallic alloys, with different vanadium contents, were obtained by co-sputtering of vanadium and titanium targets on glass and silicon substrates. The films were characterized by means of scanning electron microscopy (SEM), x-ray diffraction, Raman spectroscopy, infrared spectroscopy, x-ray photoelectron spectroscopy, and UV–Vis spectroscopy. In addition, photodegradation of methylene blue (MB) in aqueous solution, on these composite films, under visible light, was also evaluated. SEM images show nanostructured films with nanospherical or nanobaranched surface morphologies. Furthermore, the results reveal that at low vanadium (V) content, the films consist of one or mixture of amorphous phases (TiVOx or TiOx plus VOx, or both). In the case of films with higher V content, two separated phases, namely V2O5 and amorphous TiOx are obtained. The UV–Vis analyses demonstrate a redshift of optical absorption over a broad band of optical frequencies for films with high V content. Furthermore, these films manifested a good catalytic activity toward MB degradation with degradation rate of 78% after 120 min of visible light irradiation. Such composite thin films can be promising not only for photocatalytic applications, but also for photonic and sensing devices.

Sixth Generation (6G) Cognitive Radio Network (CRN) Application, Requirements, Security Issues, and Key Challenges

Recently, 5G installation has been started globally. Different capabilities are in the consistent procedure, like ultrareliability, mass connectivity, and specific low latency. Though, 5G is insufficient to meet all the necessities of the future technology in 2030 and so on. Next generation information and communication technology is playing an important role in attraction of researchers, industries, and technical people. With respect to 5G networks, sixth‐generation (6G) CR networks are anticipated to familiarize innovative use cases and performance metrics, such as to offer worldwide coverage, cost efficiency, enhanced spectral, energy improved intelligence, and safety. To reach such requirements, upcoming 6G CRNs will trust novel empowering technologies. Innovative network architecture and transmission technologies and air interface are of excessive position, like multiple accesses, waveform design, multiantenna technologies, and channel coding schemes. (1) To content, the condition should be of worldwide coverage, there will be no limit on 6G to global CR communication networks that may require to be completed with broadcast networks, like satellite communication networks, therefore, attaining a sea integrated communication network. (2) The spectrums overall will be entirely travelled to the supplementary rise connection density data rates in optical frequency bands, millimeter wave (mmWave), sub‐6 GHz, and terahertz (THz). (3) To see big datasets created because of tremendously varied CR communication networks, antenna rush, diverse communication scenarios, new provision necessities, wide bandwidth, and 6G CRNs will allow an innovative variety of intelligent applications with the assistance of big data and AI technologies. (4) Need to improve network security when deploying 6G technology in CR networks. 6G is decentralized, intended, intelligent innovative, and distributed network. In this article, we studied a survey of current developments and upcoming trends. We studied the predicted applications, possible technologies, and security issues for 6G CR network communication. We also discussed predicted future key challenges in 6G.

A Nanoplasmonic Branchline Coupler for Subwavelength Wireless Networks

This article presents the new nanoplasmonic branchline coupler (BLC) by using an edge coupled coplanar waveguide (CPW) with a finite length suspended backed conductor. The proposed coupler is designed by using the theoretical transmission line (TL) method, which is established based on the basic characteristics of the related CPW structures. For this purpose, the normalized propagation constant, propagation length, and characteristic impedance of the conductor backed coplanar waveguide (CBCPW) structure are studied and analyzed by applying a fullwave analysis using CAD simulation software tool CST microwave studio suite. Mainly, the coupling coefficient of the BLC can be controlled by changing the length of the backed conductor. In this article, the BLC is designed and analyzed by using a finite length backed conductor based on the theoretical TL method at a 3-dB coupling coefficient. This newly designed coupler can operate at two arbitrary optical frequency bands (O-band and L-band). Hence, it allows us to exchange many other couplers with their narrow bandwidths using this branchline coupler for subwavelength nanoscale wireless network applications and photonic integrated circuits (PICs).

A theoretical investigation on reciprocity-inspired wide-angle spectrally-selective THz absorbers augmented by anisotropic metamaterials

In this paper, a theoretical framework relying on the reciprocity theorem is proposed to accurately design a spectrally-selective THz superstrate-loaded metamaterial absorber (SLMA) exhibiting wide-angle feature. By leveraging high-order Floquet harmonics in a generalized transmission line model characterizing the conventional metamaterial absorbers (MAs), it is demonstrated that MAs suffer from impedance mismatch, especially at near grazing angles. From an impedance matching viewpoint, this major challenge is tackled in this paper via two different designs, exploiting a magneto-electric anisotropic Huygens' metamaterial and a multilayer dielectric structure at a certain distance over the MA plane. The numerical results corroborate well the theoretical predictions, elucidating that the proposed SLMA significantly broadens the angular performance of the MA up to near grazing angles (about 80°), where high absorptivity is still achieved in both principal planes. The deteriorating effect of diffraction modes has been comprehensively analyzed. In comparison to the previous wide-angle MA reports based on intricate particle geometries and brute-force optimizations, the proposed design features a straightforward semi-analytical algorithm, which can also be re-developed for microwave, mid-infrared, and optical frequency bands and for any type of MA element. The proposed SLMA would be very promising for various wavelength-selective applications such as sensors and imaging.

Electromagnetic Characterization of Graphene-Plasma Formations

This article presents an efficient electromagnetic (EM) characterization technique to extract the permeability and permittivity tensors of multilayer graphene-plasma formations. This method uses the scattering parameters of normally incident circularly polarized plane waves in free space, making use of the circular polarization to deal with the discontinuity ambiguities in the extracted tensorial permeabilities and permittivities. The proposed method directly determines the linear permeability and permittivity tensors from the combined parameters that are deduced from the circularly polarized wave components. This simple procedure succeeds in overcoming the limitation to usable terahertz and optical frequency bands suffered by existing extraction techniques for lossy and lossless anisotropic material behavior. The retrieved graphene-plasma tensor elements are validated by means of comparison with analytical parameters obtained by solving the original EM problem using the transfer matrix method extended for the circularly polarized fields.

Coupling effects in dielectric metamaterials

Dielectric metamaterials have been proved to have broad applications in the microwave and optical frequency bands. However, as the components of the communication system shrink, the coupling effects caused by tight arrangements between the dielectric structures have not been carefully studied enough yet. In this paper, the deep coupling effects in limited space between the adjacent dielectric metamaterials cubes are investigated. According to the correlation between the coupling effect and the incident wave, the influence of coupling effect on the scattering characteristics of the structure is studied from three aspects: coupling effect parallel to the wave vector/electric field/magnetic field, respectively. Field distributions and constitutive parameters are also demonstrated to illustrate the relationship between the coupling effects and scattering coefficients.

Green RF/FSO Communications in Cognitive Relay-Based Space Information Networks for Maritime Surveillance

In this paper, we provide an overview of a space-based network for the maritime surveillance information sharing and evaluate the coverage, the throughput and the real-time reception capacity of the presented systems. The focus is on the feeder link of a multi-hop geostationary satellite system. We propose a cognitive relay-based architecture for the inter-satellite and satellite-to-ground communication with a multi-band spectrum sensing operating on both free-space optical (FSO) and radio frequency (RF) bands. Furthermore, modeling the spectrum sensing energy as a Gamma distribution we derive the formula of the network energy consumption in presence of multipath fading and considering two different data relay schemes: amplify-and-forward (AF) and decode-and-forward (DF). We prove that the RF/FSO communication in the proposed relay-based architecture enhances the data transmission increasing the throughput and the real-time reception capacity and reducing the network energy consumption. The performance evaluation shows that the DF scheme overcomes the AF one in terms of energy consumption while the AF scheme overcomes the DF one in terms of probability detection, both experience the same throughput. Finally, the proposed novel architecture has been compared with existing ones showing that it enhances the current maritime surveillance systems by jointly optimizing communication functionalities.

Engineering the dispersion properties of multilayered periodic segmented waveguides and nanowire waveguides

We proposed and systematically analyzed the propagation properties of periodic segmented waveguides and nanowire waveguides where the segments and nanowires have been considered to be composed of multilayers of silicon and silica with subwavelength thickness. We demonstrated, through a comprehensive number of numerical simulations involving modal analysis, that the artificial uniaxial negative anisotropy introduced by the multilayered segments or nanowire can be used to engineer waveguides with independent polarization propagation characteristic over the entire band of optical communication frequencies.

On-chip magnetic-free optical multi-port circulator based on a locally linear parity–time symmetric system

This paper introduces a new type of magnetic-free optical circulator without utilizing a Faraday rotator, polarization beam splitter, and magneto-optic material for the first time, to the best of our knowledge. The designed circulator operates linearly and relies on the parity-time symmetric (PTS) system. The property of the non-Hermitian system (the linear PTS system) plays the key role in the suggested optical circulator so that it can simultaneously support N-ports. The proposed device is integrated, broadband, and operates in the optical telecommunication frequency band. A great value of isolation rate of 47dB is achieved. The proposed circulator can pave the way for optical integrated circuits and optical networks.